The present invention relates generally to the field of printing presses, and specifically to a method and apparatus for visually inspecting images on a substrate moving along a printing press using a complementary metal oxide semiconductor (“CMOS”) based image recording device and/or a light emitting diode (“LED”) illumination source.
In an exemplary printing press such as a web offset press, a web of material, typically paper, is fed from a storage mechanism, such as a reel stand, to one or more printing units that imprint the web with repetitive images. The imprinted web is typically driven through a number of processing units such as a dryer unit, a chill stand, and/or a coating machine. The web is then fed to a former/folder.
Various conditions of the printing press (e.g., web tension, presence of splices, and influence from folders, slitters, imprinters, gluers, and other processing equipment) may cause the position of the web to vary over time with respect to the processing stations (i.e., printing units, processing units, former/folder, etc.). Accordingly, it is necessary to periodically adjust the positional relationship of the web and the processing stations by advancing or retarding the longitudinal position of the web and/or adjusting the lateral position of the web.
Control systems that control the adjustment of the positional relationship of the web and the processing stations are generally known and include cutoff control. Typically, the amount of positional adjustment is determined by observing the movement of the web using a visual inspection system and/or using a printing press operator manually observing the web. Other printing press control systems include color registration, color control and web inspection.
Existing visual inspection systems that operate in conjunction with control systems typically utilize at least one camera assembly. Camera assemblies typically include an image recording device, such as a charge-coupled device (“CCD”) camera. The camera assemblies also typically include an illumination system for illuminating the field of view of the image recording device when an image is being recorded. Existing illumination systems include a light source such as a pulsed xenon strobe light and/or an incandescent light.
Generally, each camera assembly used in a visual inspection system is coupled to a dedicated processing unit (i.e., each processing unit accommodates only a single camera assembly) that is thereby coupled to a control system used to control an aspect of the printing press. At least a portion of the control system may be included in the dedicated processing unit. Technical requirements of the existing visual inspection systems generally necessitate that the interconnection that couples a camera assembly to the dedicated processing unit is less than a maximum fifteen foot distance. Existing camera assemblies are typically synchronized to the traveling web using a series of shaft encoders. Existing camera assemblies do not include the ability to record every revolution or iteration of the traveling web (i.e., the camera assemblies do not include sampling rates that are high enough to record at least a portion of an image printed on the traveling web), and thus existing camera assemblies rely on sampling techniques to analyze the traveling web for movement. Existing visual inspection systems cannot detect variation in the position of the web in any direction that is not in the same plane as the primary web movement.
The light sources utilized in the illumination system of existing visual inspection systems generally produce heat that must be dissipated to reduce adverse effects from the heat on the image quality (e.g., reduced image quality due to lens distortion). Additionally, the light sources would preferably use less power, cost less, and last longer.